1. Field of the Invention
The present invention pertains to the field of liquid precursor solutions that may be used to produce solid metal oxide materials having specialized electrical properties. More specifically, the preferred precursor solutions include an octane solvent mixed with polyoxyalkylated metal complexes of a type that may be used in liquid deposition processes for manufacturing thin-film electrical components and, especially, ferroelectric or dielectric materials for use in integrated circuits.
2. Statement of the Problem
The use of hazardous chemicals in manufacturing processes has commensurate environmental and civil liability risks; however, these chemicals often continue to be used because no suitable replacement can be found. Even where replacement chemicals can be employed, manufacturers often choose to continue use of the old chemicals because of perceived quality and reliability problems in changing the manufacturing process. For example, the commercially available xylene solvents are typically a mixture of ortho, para, and meta xylene isomers. As of 1975, xylene was the 26th highest-volume produced chemical in the United States, and large xylene quantities are still produced by fractional distillation from petroleum. Nevertheless, human health-safety concerns have led government agencies to establish a safe threshold limit value for workplace exposure to xylene at about 100 ppm. Xylene, which typically has a flash point ranging from about 27.2 to 46.1.degree. C. (values may vary depending upon the grade of xylene), also constitutes a dangerous fire hazard. Similar problems exist with the use of alcohols, ethers, esters and ketones, as well as with other aromatic hydrocarbons in addition to xylene.
Metal oxide films for use in integrated circuits have most frequently been formed by conventional sputtering techniques. See for example, Kuniaki Koyama, et al., "A Stacked Capacitor With (Ba.sub.x Sr.sub.1-x)TiO.sub.3 For 256 M DRAM" in IDEM (International Electron Devices Meeting) Technical Digest, December 1991, pp. 32.1.1-32.1.4, and U.S. Pat. No. 5,122,923 issued to Shogo Matsubara et al. Other fabrication methods include pulsed laser deposition, and rapid quenching as listed in Joshi, P. C. et al., "Structural and Optical Properties of Ferroelectric Thin Films By Sol-gel Technique," Appl. Phys. Lett., Vol 59, No. 10, November 1991. These methods are relatively violent processes and, thus, inherently result in relatively poor control of the composition of the final thin film as a whole and variable composition throughout the film.
Metal oxide films have also been formed from sol-gels, i.e., a metal alkoxide which is hydrated to form a gel. These gels are applied to a semiconductor substrate to form a film, and then decomposed to form a metal oxide. One such method comprises the application of a sol-gel to a substrate followed by heat treatment. The heat decomposes the sol-gel and drives off the organics to form the metal oxide. See for example, U.S. Pat. No. 5,028,455 issued to William D. Miller et al., the Joshi article cited above, and B. M. Melnick, et al., "Process Optimization and Characterization of Device Worthy Sol-Gel Based PZT for Ferroelectric Memories", in Ferroelectrics, Vol 109, pp. 1-23 (1990). In another method, what has been termed a "MOD" solution is applied to a substrate followed by heating which decomposes the MOD solution and drives off the organics to form the metal oxide. See "Synthesis of Metallo-organic Compounds for MOD Powers and Films", G. M. Vest and S. Singaram, Materials Research Society Symposium Proceedings, Vol. 60, 1986 pp. 35-42 and "Metalorganic Deposition (MOD): A Nonvacuum, Spin-on, Liquid-Based, Thin Film Method", J. V. Mantese, A. L. Micheli, A. H. Hamdi, and R. W. Vest, in MRS Bulletin, October 1989, pp. 48-53. Generally the sol-gel method utilizes metal alkoxides as the initial precursors, while the MOD technique utilizes metal carboxylates as the initial precursors. These techniques require the addition of water to the solution prior to application of the solution to a substrate. The use of water induces undesirable chemical reactions, e.g., the possible precipitation of metalized reagents and severe viscosity changes.
The above references typically discuss precursor compounds having metals that bond with organic ligands. These ligands must be broken down and removed during the heating-decomposition process. The molecular geometry creates relatively large distances across which the metal and oxygen atoms must link to form metal oxides. These distances can often result in cracking or other imperfections in the film, and, accordingly, impose a severely burdensome manufacturing duty of exacting control over multiple parameters, such as film thickness, drying and annealing temperatures, the substrate used etc.
In other liquid deposition processes, such as the sol-gel process described in the Melnick reference, the metal-oxygen-metal bonds of the final metal oxide are present in some degree; however the precursor is highly unstable and, therefore, is unsuited for use except immediately after preparation in the laboratory. One sol-gel reference, the Miller patent referenced above, mentions one metal carboxylate, lead tetra-ethylhexanoate, as a possible precursor; however the reference fails to disclose how this substance may be used as a sol-gel, and, furthermore, rejects this precursor as being less desirable because the large organic group was thought to result in more defects in the final film.
Thin-film metal oxide electronic components, i.e., those having thicknesses of less than about ten microns, may often require ferroelectric or dielectric properties. The film should have a relatively uniform grain size, which results in better crystalline qualities such as films free of cracks and other defects. The film grain size should also be small compared to the thickness of the film; otherwise the roughness of the film can be comparable to the thickness and other dimensions of the device components, which can make it difficult or impossible to fabricate devices within performance tolerances and can result in short circuits or other electrical breakdowns. Further, it is important that the fabrication processes be performed relatively rapidly, since long processes are more expensive in terms of the use of facilities and personnel.
In integrated circuit construction, it is sometimes useful to employ materials that exhibit relatively strong ferroelectric and dielectric behavior. These materials may include perovskites, and especially ABO.sub.3 perovskites, such as barium titanate, wherein A and B are respective A and B site metal cations. In addition to having ferroelectric properties, the perovskite-like layered superlattice materials discovered by G. A. Smolenskii, V. A. Isupov, and A. I. Agranovskaya (See Chapter 15 of the book, Ferroelectrics and Related Materials, ISSN 0275-9608, [V.3 of the series Ferroelectrics and Related Phenomena, 1984] edited by G. A. Smolenskii (especially sections 15.3-15), may also have high dielectric constants. On the other hand, these types of materials are not widely used on a commercial basis due to problems with polarization fatigue and retention of the ferroelectric polarization state. These problems are thought to result from uncompensated defects in the ferroelectric crystalline structure and associated ionic charge migrations.
Integrated circuits, which are sometimes called semiconductor devices, are generally mass produced by fabricating hundreds of identical circuit patterns on a single wafer. This wafer is subsequently sawed into hundreds of identical dies or chips. While integrated circuits are commonly referred to as "semiconductor devices" they are in fact fabricated from various materials which are either electrically conductive, electrically non-conductive, or electrically semiconductive.
The material out of which the wafer and other parts of integrated circuits are fabricated is generally either silicon (Si) or gallium arsenide (GaAs). Silicon is the most commonly used material, and the present invention will be described in terms of silicon technology. Nevertheless, the invention is also applicable to semiconductor technologies based on GaAs or even other semiconductors. Silicon can be used in either the single crystal or polycrystalline form in integrated circuits. In the integrated circuit fabrication art, polycrystalline silicon is usually called "polysilicon" or simply "poly", and will be referred to as such herein. Both forms of silicon may be made conductive by the addition of impurities, which are commonly referred to as "dopants." If the dopant is an element such as boron which has one less valence electron than silicon, electron "holes" become the dominant charge carrier and the doped silicon is referred to as p-type silicon. If the doping is with an element such as phosphorus which has one more valence electron than silicon, additional electrons become the dominant charge carriers and the doped silicon is referred to as n-type silicon.
Silicon dioxide is commonly used as an insulator or barrier layer in silicon-based semiconductors devices. Its use is so universal that in the integrated circuit art it is often referred to as simply as "oxide". Another common silicon-based structure is called polycide. This is a composite, layered material comprising a layer of metal silicide and a layer of polysilicon. CMOS (Complimentary Metal Oxide Semiconductor) technology is currently the most commonly used integrated circuit technology, and thus the present invention will be described in terms of silicon-based CMOS technology, although it is evident that the invention may be utilized in other integrated circuit technologies.
3. Solution to the problem:
The present invention overcomes the problems that are outlined above by providing an octane solvent for use in combination with essentially water-free polyoxyalkylated metal complexes that form liquid precursor solutions capable of yielding solid metal oxides suitable for use in integrated circuits and other electronic components incorporating thin-film metal oxides.
The related application Ser. No. 07/807,439 indicated that a xylene solvent is most preferred for use in liquid polyoxyalkylated metal precursor solutions, except where the respective polyoxyalkylated metal portions of these solutions include strongly polar molecules in liquid solution. It has been discovered that certain alkane solvents are capable of providing results on par with xylene solvents, thus, enabling avoidance of the deleterious effects of earlier solvents on human health and the environment. In particular, as compared to xylene, an octane solvent produces similar results in the manufacture of complex metal oxide electrical components, but octane is even more preferred for its relatively decreased levels of activity as an agent for causing environmental damage, its reduced incidence of health problems in exposed workers, and lessened safety risks such as potential for explosions or flash fires. The n-octane solvent is nearly ideal for liquid metal oxide deposition processes due to its high boiling point (125-127.degree. C.) and apolar solvent properties that are similar to xylene.
Broadly speaking, the present invention includes methods and materials pertaining to the use of a precursor solution for forming complex metal oxides, wherein the oxide-forming fractions are dissolved in an alkane solvent. The solid complex metal oxides that are formed from these liquid precursors preferably have ferroelectric and/or dielectric properties that render the metal oxides useful in thin-film electronic devices such as integrated circuits. The alkane solvent preferably has from seven to ten carbons providing simple alkane or cycloalkane structures. For purposes of keeping these solutions essentially free of water by fractional distillation, the alkane solvent boiling point ("b.p.") should be greater than that of water; a straight-chain heptane (b.p.=98.4.degree. C.) would not quite be suitable for use where distillation of water portions is required, whereas cycloheptane (b.p.=117.degree. C.) would be suitable. Even so, straight-heptane would be suitable for distillation of solution liquids having a boiling point less than that of heptane. The alkane solvent is more preferably an octane, which is even more preferably essentially unsubstituted and unbranched, i.e., n-octane.
The metal oxide-forming portions of these solutions are preferably comprised of a metal polyoxyalkylated complex including a metal moiety bonded with an oxyorganic ligand selected from a group consisting of alkoxides, carboxylates, and mixtures thereof.
Prior related applications have recited that a xylene solvent works well in most precursor solutions. Where highly electropositive elements are present in the solutions, the solvent preferably includes 2-methoxyethanol, -butyl acetate and/or excess 2-ethylhexanoic acid. Some additional solvents that may be used, together with their boiling points, include: alcohols, such as 1-butanol (117.degree. C.), 1 -pentanol (117.degree. C.), 2-pentanol (119.degree. C.), 1-hexanol (157.degree. C.), 2-hexanol (136.degree. C.), 3-hexanol (135.degree. C.), 2-ethyl-1-butanol (146.degree. C.), 2-methoxyethanol (124.degree. C.), 2-ethoxyethanol (135.degree. C.), and 2-methyl-1-pentanol (148.degree. C.); ketones, such as 2-hexanone (methyl butyl ketone) (127.degree. C.), 4-methyl-2-pentanone (methyl isobutyl ketone) (118.degree. C.), 3-heptanone (butyl ethyl ketone) (123.degree. C.), and cyclohexanone (156.degree. C.); esters, such as butyl acetate (127.degree. C.), 2-methoxyethyl acetate (145.degree. C.), and 2-ethoxyethyl acetate (156.degree. C.); ethers, such as 2-methoxyethyl ether (162.degree. C.) and 2-ethoxyethyl ether (190.degree. C.); and aromatic hydrocarbons, such as xylenes (138.degree. C.-143.degree. C.), toluene (111.degree. C.) and ethylbenzene (136.degree. C.).
The precursor solutions may be used to form a ferroelectric or dielectric perovskite-like periodically repeating layered superlattice material, as described in copending application Ser. No. 07/965,190, which is hereby incorporated by reference herein. These layered superlattice materials comprise complex oxides of metals, such as strontium, calcium, barium, bismuth, cadmium, lead, titanium, tantalum, hafnium, tungsten, niobium zirconium, bismuth, scandium, yttrium, lanthanum, antimony, chromium, and thallium that spontaneously form layered superlattices, i.e. crystalline lattices that include alternating layers of distinctly different sublattices. Generally each layered superlattice material will include two or more of the above metals, and can be described in terms of a single average formula; for example, barium, bismuth and niobium form the layered superlattice material barium bismuth niobate, BaBi.sub.2 Nb.sub.2 O.sub.9.
The layered superlattice materials may be summarized more generally under the formula: EQU (1) A1.sub.w1.sup.+a1 A2.sub.w2.sup.+a2 . . . Aj.sub.wj.sup.+aj S1.sub.x1.sup.+s1 S2.sub.x2.sup.+s2 . . . Sk.sub.xk.sup.+sk B1.sub.y1.sup.+b1 B2.sub.y2.sup.+b2 . . . Bl.sub.yl.sup.+bl Q.sub.z.sup.-2,
where A1, A2 . . . Aj represent A-site elements in the perovskite-like structure, which may be elements such as strontium, calcium, barium, bismuth, lead, and others S1, S2 . . . Sk represent super-lattice generator elements, which usually is bismuth, but can also be materials such as yttrium, scandium, lanthanum, antimony, chromium, thallium, and other elements with a valence of +3, B1, B2 . . . Bl represent B-site elements in the perovskite-like structure, which may be elements such as titanium, tantalum, hafnium, tungsten, niobium, zirconium, and other elements, and Q represents an anion, which generally is oxygen but may also be other elements, such as fluorine, chlorine and hybrids of these elements, such as the oxyfluorides, the oxychlorides, etc. The superscripts in formula (1) indicate the valences of the respective elements, and the subscripts indicate the number of moles of the material in a mole of the compound, or in terms of the unit cell, the number of atoms of the element, on the average, in the unit cell. The subscripts can be integer or fractional. That is, formula (1) includes the cases where the unit cell may vary throughout the material, e.g. in Sr.sub.0.75 Ba.sub.0.25 Bi.sub.2 Ta.sub.2 O.sub.9, on the average, 75% of the time Sr is the A-site atom and 25% of the time Ba is the A-site atom. If there is only one A-site element in the compound then it is represented by the "A1" element and w2 . . . wj all equal zero. If there is only one B-site element in the compound, then it is represented by the "B1" element, and y2 . . . yl all equal zero, and similarly for the superlattice generator elements. The usual case is that there is one A-site element, one superlattice generator element, and one or two B-site elements, although formula (1) is written in the more general form since the invention is intended to include the cases where either of the sites and the superlattice generator can have multiple elements. The value of z is found from the equation: EQU (2) (a1w1+a2W2 . . . +ajwj)+(s1x1+s2x2 . . . +skxk)+(b1y1+b2y2 . . . +bjyj)=2z.
Formula (1) includes all three of the Smolenskii type compounds. The layered superlattice materials do not include every material that can be fit into the formula (1), but only those which spontaneously form themselves into crystalline structures with distinct alternating layers. It should be noted that the x, y, and z symbols in the formula (1) should not be confused with the x, y, and z, symbols used in the Formulae (3) and (4). The formula (1) is a general formula for layered superlattice materials, while the formulae (I) and (I') are formulae for solid solutions of particular layered superlattice materials.
It should also be understood that the term layered superlattice material herein also includes doped layered superlattice materials. That is, any of the material included in formula (1) may be doped with a variety of materials, such as silicon, germanium, uranium, zirconium, tin or hafnium. For example, strontium bismuth tantalate may be doped with a variety of elements as given by the formula: EQU (3) (Sr.sub.1-x M1.sub.x)Bi.sub.2 (Nb.sub.1-y M2.sub.y)O.sub.9 +.alpha. M30,
where M1 may be Ca, Ba, Mg, or Pb, M2 may be Ta, Bi, or Sb, with x and y being a number between 0 and 1 and preferably 0.ltoreq..times..ltoreq.0.2, 0.ltoreq.y .ltoreq.0.2, M3 may be Si, Ge, U, Zr, Sn, or Hf, and preferably 0.ltoreq..alpha..ltoreq.0.05. Materials included in this formula are also included in the term layered superlattice materials used herein.
Similarly, a relatively minor second component may be added to a layered superlattice material and the resulting material will still be within the invention. For example, a small amount of an oxygen octahedral material of the formula ABO.sub.3 may be added to strontium bismuth tantalate as indicated by the formula: EQU (4) (1-x) SrBi.sub.2 Ta.sub.2 O.sub.9 +xABO.sub.3,
where A may be Bi, Sr, Ca, Mg, Pb, Y, Ba, Sn, and Ln; B may be Ti, Zr, Hf, Mn, Ni, Fe, and Co; and x is a number between 0 and 1, preferably, 0.ltoreq..times..ltoreq.0.2.
Likewise the layered superlattice material may be modified by both a minor ABO.sub.3 component and a dopant. For example, a material according to the formula: EQU (5) (1-x) SrBi.sub.2 Ta.sub.2 O.sub.9 +XABO.sub.3, +.alpha. MeO,
where A may be Bi, Sb, Y and Ln; B may be Nb, Ta, and Bi; Me may be Si, Ge, U, Ti, Sn, and Zr; and x is a number between 0 and 1, preferably, 0.ltoreq..times..ltoreq.0.2, is contemplated by the invention.
Alternatively, the precursor solutions may be used to form ABO.sub.3 perovskites of the type described in copending application Ser. No. 08/132,744 filed Oct. 6, 1993, which is hereby incorporated by reference herein. The ferroelectric materials are preferably perovskites, and ABO.sub.3 perovskites are particularly preferred. The ABO.sub.3 perovskites are materials of the general form ABO.sub.3 where A and B are cations and O is an oxygen anion component. The ABO.sub.3 term also includes materials were A and B represent multiple elements, e.g., materials of the form A'A"BO.sub.3, AB'B"O.sub.3, and A'A"B'B"O.sub.3, where A', A", B' and B" are different metal elements. The A, A', and A" metals are more preferably selected from the group of metals consisting of Ba, Bi, Sr, Pb, Ca, and La. The B, B', and B" metals are more preferably selected from the group consisting of Ti, Zr, Ta, Mo, W, and Nb. Many of these ABO.sub.3 perovskites are ferroelectrics, though some that are classed as ferroelectrics may not exhibit ferroelectricity at room temperature. These ferroelectric materials typically have relatively high dielectric constants, and are useful in high dielectric constant capacitors, whether or not they are ferroelectric. Lead zirconate titanate, strontium titanate, and, especially, barium strontium titanate ("BST") are most preferred for use as the ferroelectric material.
The preferred process for preparing polyoxyalkylated metal precursors is provided in copending application Ser. No. 08/132,744 filed Oct. 6, 1993, now U.S. Pat. No. 5,514,822 which is hereby incorporated by reference herein. The process preferably includes reacting a metal, such as barium, strontium, or titanium, with an alcohol (e.g., 2-methoxyethanol) to form a metal alkoxide, and reacting the metal alkoxide with a carboxylic acid (e.g., 2-ethylhexanoic acid) to form a metal alkoxycarboxylate or metal alkoxide having a --O--M--O--M--O-- group according to one of the generalized Formulae: EQU (6) (R'--COO--).sub.a M(--O--R).sub.n, or
(7) (R'--C--O).sub.a --M--(O--M'--(O--C--R").sub.b-1).sub.n,
wherein M is a metal having an outer valence of (a+n) and M' is a metal having an outer valence of b, with M and M' preferably being independently selected from the group consisting of tantalum, calcium, bismuth, lead, yttrium, scandium, lanthanum, antimony, chromium, thallium, hafnium, tungsten, niobium, zirconium, manganese, iron, cobalt, nickel, magnesium, molybdenum, strontium, barium, titanium, and zinc; R' is an alkyl group having from 4 to 15 carbon atoms and R is an alkyl group having from 3 to 9 carbon atoms. Of course, even though the use of substances according to Formulae (6) and (7) are more preferred, mixtures of metal alkoxides, metal carboxylates, and metal alkoxycarboxylates in any proportion are acceptable for use in precursor solutions.